To begin with, STAAD.Pro does not design flat bars. The only way to get a design out of STAAD for these type of sections is to define these as User table General Sections ( using menu option Tools > Create User table > New table and choosing section type as General ). You would need to enter the appropriate properties of the section like Cross Sectional Area, Moment of Inertia, Section Modulii and others. STAAD would then design the user table general section following rules applicable for wide flanges and generate unity ratios.